Organic dyestuffs are well known in the prior art as having the capacity to sensitize materials such as silver halides, zinc oxide, etc. and are therefore useful for photographic purposes; typically such dyestuffs are cyanine dyes, merocyanine dyes, azacyanine dyes, methine dyes, etc., see e.g. French Pat. No. 2,137,985, Canadian Pat. No. 849,450, U.S. Pat. No. 3,597,196, U.S. Pat. No. 3,110,591, etc. However, such dyestuffs, although they are good photoconductors, are very poor electrical conductors and therefore are reported to make relatively poor photovoltaic devices, i.e. a device which results in the direct conversion of light to electrical current (see "Solar Cells", National Academy of Sciences, 1972, p. 17).
Thus far in the prior art, when using such dyestuffs in photovoltaic devices, it has been found necessary to use them in combination with materials which form heterojunctions. That is, such materials are known to have the capacity to become sensitized or de-sensitized when placed in contact with these dyestuffs and exposed to light; typical examples of such materials are silver halides, titanium dioxide, zinc oxide, cadmium sulfide, selenium and polyvinylcarbazole.
In contrast to the prior art photovoltaic devices relying on the formation of heterojunctions, the present invention relies on the use of the same dyestuffs to form a Schottky barrier at the interface between the dyestuff and the electrode comprised of the element having a work function equal to or less than that of aluminum. That is, the materials such as silver halides, titanium dioxide, etc. described above are not used in contrast with these dyestuffs. Rather, certain elements, more particularly described below, are utilized as electrode materials in contrast with these dyestuffs and the resultant photovoltaic devices convert light (in the visible spectrum) into levels of electrical current which are orders of magnitude greater (e.g. hundreds of times greater) than that generated by photovoltaic devices relying on the formation of heterojunctions (e.g. see "p-n Junctions Between Organic and Inorganic Photoconductors" by H. Meier et al, Ber. Bunsenges Physik. Chem., Volume 69, #2, pp. 160-167 (1965)).
In U.S. Pat. No. 3,530,007, Golubovic describes a solar cell in which the organic compound (e.g. a dyestuff) is in ohmic contact with both electrodes. In the instant invention, a non-ohmic contact is formed between the dyestuff and the electrode comprised of the element having a work function equal to or less than that of aluminum (thus giving rise to a Schottky barrier) and an ohmic contact is formed between the opposite surface of the dyestuff and the electrode comprised of the element having a work function greater than that of aluminum. It has been unexpectedly found that the particular organic compounds utilized in this invention (which differ from those utilized by Golubovic) result in high efficiencies (the Golubovic-type devices give low efficiencies), notwithstanding that such high efficiencies result from Schottky barrier formation (it generally being thought necessary to generate heterojunctions to obtain high efficiencies).